This study involves an investigation of the neuroendocrine and pharmacological regulation of the pituitary gland and pituitary hormones. We have noted very different sensitivities of trypsin-dispersed pituitary cells from normal glands and tumors to dopamine agonists. The decreased sensitivity of cells of the MtTW15 and 7315 alpha tumors to bromocriptine suggested a decreased sensitivity of dopamine receptors in tumors or that the tumor cells contained fewer dopamine receptors. The latter seems to be the case. Since spontaneous tumors in rats, in contrast to the MtTW15 and 7315 alpha tumors which have been transplanted for 20 years, seem not to respond to dopamine agonists, we plan to repeat our studies of dopamine agonist sensitivity in rats with spontaneous pituitary tumors. These studies will follow the in vivo administration of bromocriptine and in addition study the sensitivity of dispersed pituitary cells to the drug. We also plan to study the affinity of the cells for dopamine agonists with respect to binding studies and the number of receptors. The in vitro sensitivity of normal pituitary cells obtained from rats which have pituitary tumors will also be studied to determine whether the autofeedback of prolactin on its own secretion occurs here as it does in rats bearing transplanted tumors.